The present invention relates to a process for a pretreatment of a polyolefin product before coating, and more particularly, to a pretreatment which permits a significant improvement of the adhesion of coating film.
Polyolefin products, such as polyethylene (hereinafter referred to as "PE") or polypropylene (hereinafter referred to as "PP"), are inexpensive thermoplastic resins which are excellent in chemical resistance and mechanical properties and are widely used in various fields. Polyolefin products, however, have not been widely applied to articles to be coated because of low surface activities and adverse adhesion of coating film.
In the prior art, polyolefin incorporated with an inorganic filler or rubber modifying material was proposed in order to improve the adhesion of coating film. Such incorporation did not always yield satisfactory adhesion and was, therefore, not practicable for automobile parts which are subjected to strict conditions of heat resistance or resistance to gasoline in the adhesion of coating film.
The present invention previously tried to improve the adhesion of coating film of a polyolefin product by performing a plasma treating on the surface of a polyolefin product before coating in the presence of oxygen etc. and activating the surface.
Such process, however, has disadvantages in that a long treating time is required to yield a satisfactory adhesion using only plasma treating and in that the products are apt to be contaminated by dust or oil in manufacturing, resulting in unevenness of the adhesion of coating film in each product.